1. Field of the Invention
This invention is in the field of foil type papermaking machine screens and is designed to reduce the generation of abrupt pressure pulses which eventually manifest themselves in a non-homogeneous distribution of fibers from the headbox. Specifically, the present invention provides an improved foil type screen assembly including a plurality of rotating foils which cooperate with a helical slot provided in the housing to reduce the amplitude of discrete pressure pulses which are frequently encountered in foil type screens.
2. Description of the Prior Art
Stock screens are used in the papermaking process for at least partially cleaning the stock before it flows to the headbox where it is dewatered to form a paper web. Such stock screens are conventionally tubular in shape with the stock being directed either to the inner surface or the outer surface of the tubular screen with the desired fibers flowing through the screen and the rejected stock material including sheaves, particles, dirt, knots, and like being rejected by the screen and being removed through a reject line. In the case where the supply stock is delivered to the inside of the screen and the stock flows outwardly through the screen, the stock to be screened may be admitted by flowing in at one end of the tubular screen, and the rejects flow out of the other end. The acceptable material is received by a discharge from an annular chamber about the screen. Such devices are shown, for example, in U.S. Pat. No. 4,111,799 directed to a horizontally disposed stock screen assembly, and U.S. Pat. No. 4,126,513 directed to a vertically extending stock screen assembly. In each of the assemblies described in these patents, stock pulsing foils extend substantially the entire effective length of the screen. The foils rotate at a uniform velocity over the surface of the screen to cause relatively low frequency, high amplitude pressure pulsations. Such pressure pulsations are necessary for the satisfactory operation of the screen, but they introduce corresponding pressure pulsations in the stock delivery to the headbox. Such pulsations, as previously noted, disturb uniform distribution of the stock and have an adverse effect on flow of the stock through the slice opening onto the web-forming surface. It is essential for satisfactory high speed operation and good formation that the pulse fluctuations be substantially eliminated or produced at a satisfactory high frequency, to avoid machine direction instability of the stock flow onto the web-forming surface such as a fourdrinier wire and to at least reduce the possibility of causing a basis weight variation in the machine direction.
A further improvement in the foil type papermaking machine screen has been directed and claimed in U.S. Pat. No. 4,213,823. The apparatus described in this patent uses a set of staggered foils which extend from opposite ends of the screen and have their inner end portions alternating in circumferentially spaced but axially overlapping relation. The frequency of the hydraulic pulses transmitted to the outlet is increased, but the total amplitude of the foil pulses transmitted to the outlet pipe is reduced without significantly changing the horsepower requirements for driving the foil.
U.S. Pat. No. 3,261,468 describes a screen cleaning device for fibrous pulp suspensions consisting of at least one elongated foil supported for movement in a circular path about the axis of the screen and across the face thereof, the clearance between the foil and the adjacent screen face varying from the supported end to the free end of the foil so that as the foil bends under the influence of centrifugal force, it serves to clean the screen.
U.S. Pat. No. 3,713,541 relates to a machine for screening liquid fiber suspensions wherein the screen is provided with elongated slots disposed along the flow path of the suspension, the slots being arranged in a series with the slots in any given series being parallel to each other but at substantial angles to slots located in other series.
Screening apparatus utilizing a rotor which has a cylindrical wall having elevations for producing positive and negative pulse pressure thrusts is described in U.S. Pat. No. 4,188,286.
A blade type classifying apparatus for a stock suspension is described in U.S. Pat. No. 4,193,865. This patent describes a screen drum including inclined blades which bear against the screen drum at an angle to the axis of the drum and serve to sweep the surface of the drum during rotation.
In U.S. Pat. No. 4,252,641 there is described a screen assembly in which the axes of the cylindrical screen plate and the casing surrounding the plate are offset with respect to each other to form an enlargement in the annular chamber surrounding the screen plate. A partition is provided in the enlargement extending from the inlet opening to the opening at which the rejects appear. A baffle is provided which circles the screen plate helically and is substantially coextensive with the partition to provide a passage of decreasing cross-sectional area as it approaches the outlet for rejects.